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Jimmy Fallon joined "Saturday Night Live" in 1998 as a virtually unknown 24-year-old improv performer after an audition that included a spot-on impression of Jerry Seinfeld, then the brightest comedy star in the NBC universe.

Some 15 years later, that's the role Fallon occupies as he gets ready to leave "Late Night" for "The Tonight Show," completing the network’s after-hours humor trifecta – and starting, as Jay Leno and Conan O'Brien can attest, perhaps the toughest job on TV.

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With Fallon two months away from taking over the perch forever linked to Johnny Carson, he returns Saturday to his "SNL" roots as host, if not for a victory lap, then to deliver an in-person thank-you note to the show that launched his career.

During his 1998-2004 "SNL" stint, Fallon projected a buoyant energy that pulsed through his motor-mouthed one-man-band DJ Joey Mack and his raucous Massachusetts teen Sully (“Nomar!”). His bits imitating musicians – Barry Gibb on “The Barry Gibb Show” and his great man-in-mirror exchange with Mick Jagger – presaged the rock star impressions that have become a hallmark of “Late Night."

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While not an immediate “SNL” breakout star, Fallon held his own among the likes of Will Ferrell and Tina Fey, his eventual “Weekend Update” partner. It was on “Weekend Update” that Fallon showed he could directly connect with audiences just by joking with them, more or less as himself.

Fallon also distinguished himself – not always in a good way – by cracking up during skits (but then again, who among us could have kept a straight face during the “Cowbell” sketch?).

That didn’t play well on Lorne Michael’s “SNL.” But Michaels recognized that Fallon’s infectious laughter and ability to look like he’s having a good time would serve him well on “Late Night,” which he took over from O’Brien in 2009. Fallon’s style bodes positively for his upcoming shot at “Tonight,” where Carson created an atmosphere that helped viewers feel like they were guests at a nightly party, enjoying themselves with some funny and talented friends.

Fallon’s very funny and talented friend, Justin Timberlake, joins him as musical guest on Saturday's pre-Christmas installment of “SNL,” nine months after Timberlake supplied last season’s strongest hosting gig. We’ll see whether the duo offers another chapter of “The History of Rap” as Fallon preps for the challenge of making his next mark in TV history.